Steroid and nuclear receptors respond to hormones and vitamin derivatives to regulate transcriptional events critical for cell differentiation, development and homeostasis. The nuclear receptors for thyroid hormone (TR) and retinoid acid (RAR) are ligand-dependent transcriptional regulators that can activate as well as repress (silence) target gene expression. The mechanisms of such gene activation and repression are not fully understood, but are keys to elucidate mechanisms of hormone action and physiological responses to hormone and vitamin stimuli. Abnormal, constitutive repression by RAR and TR may block cell differentiation and induce oncogenic transformation. Therefore, understanding the mechanisms of transcriptional repression by the unliganded receptors will contribute to fundamental knowledge of both normal homeostasis and certain disease states. Recently, the identification and cloning of the first nuclear receptor corepressors by the applicant and others have provided novel molecular tools to dissect such gene repression events mediated by unliganded nuclear receptors. By characterizing functions of the silencing mediator for RAR and TR (SMRT), our laboratory have continued to investigate the signaling pathways mediated by the nuclear receptor-corepressor complexes. In this study, we will define and characterize the minimal receptor interacting domains of SMRT and extensively test the hypothesis that nuclear receptor interaction is essential for SMRT to function as a corepressor. We will also characterize the transcriptional repression domains of SMRT and further examine the hypothesis that transcriptional repression is also essential for SMRT to function as a corepressor. Finally, we have started and will continue to screen and characterize novel SMRT-interacting proteins by the yeast two- hybrid system. In particular, two novel, specific SMRT-interacting proteins have been identified that are likely to play important role in SMRT-nuclear receptor signaling. We strongly believe that these studies will significantly advance the fundamental knowledge in understanding molecular mechanisms of transcriptional regulation by nuclear receptors and the corepressors, and the results in provide new insights into regulation of normal hormonal responses and also hormone-related oncogenic processes and diseases.